(1) Field of the Invention
The invention relates to a process for the production of strongly adhering coatings on inorganic or organic substrates, wherein a low-temperature plasma treatment, a corona discharge treatment or a flame treatment is carried out on the inorganic or organic substrate, one or more photoinitiators are applied at normal pressure to the inorganic or organic substrate, and the substrate so precoated with photoinitiator is coated with a composition comprising at least one ethylenically unsaturated monomer or oligomer and the coating is cured by means of radiation. The invention relates also to an apparatus for carrying out the process, to the use of photoinitiators in the production of such layers and to the strongly adherent coatings themselves.
(2) Description of Related Art
The adhesion properties of coatings (e.g. finishes, paints, printing inks or adhesives) on inorganic or organic substrates, especially on non-polar substrates such as polyethylene, polypropylene or fluorine-containing polyolefins, are frequently inadequate. For that reason additional treatments have to be carried out in order to achieve satisfactory results. The adhesion can be improved by first applying special priming coatings, so-called primers, and only then applying the desired coating thereto.
A further possibility lies in exposing the substrates to be coated to a plasma treatment or corona treatment and then coating them, it being possible for a grafting process with e.g. acrylate monomers to be carried out between those two operations (J. Polym. Sci., Part A: Polym. Chem. 31, 1307-1314 (1993)).
The production of low-temperature plasmas and the plasma-assisted deposition of thin organic or inorganic layers, both under vacuum conditions and under normal pressure, have been known for some time. Fundamental principles and applications are described, for example, by A. T. Bell, “Fundamentals of Plasma Chemistry” in “Technology and Application of Plasma Chemistry”, edited by J. R. Holahan and A. T. Bell, Wiley, New York (1974) and H. Suhr, Plasma Chem. Plasma Process 3(1), 1, (1983).
It is also possible in plasmas to carry out polymerisations that result in the deposition of polymeric layers and can be used as primers. Fundamental principles and applications are described, for example, by H. Biederman, Y. Osada “Plasma Polymerization Processes” in “Plasma technology 3” edited by L. Holland, Elsevier, Amsterdam 1992.
It is also known that plastics surfaces can be subjected to a plasma treatment and as a result the subsequently applied finish exhibits improved adhesion to the plastics substrate. This is described by H. J. Jacobasch et al. in Farbe+Lack 99 (7), 602-607 (1993) for low-temperature plasmas under vacuum conditions and by J. Friedrich et al. in Surf. Coat. Technol. 59, 371-6 (1993) for plasmas ranging from in vacuo up to normal pressure conditions, the low-temperature plasma changing into a corona discharge.
A process similar to the kind mentioned at the beginning is known from WO 00/24527. That process describes the plasma treatment of substrates with immediate vapour-deposition and grafting-on of photoinitiators in vacua. A disadvantage, however, is that vapour-deposition requires the use of vacuum apparatus and, because of low deposition rates, Is not very efficient and is not suitable for industrial applications having high throughput rates.
There is a need in the art for processes for the pretreatment of substrates that can readily be carried out in practice and are not too expensive in terms of apparatus by means of which the subsequent coating of those substrates is improved.